[nci] Don't use megamorphic load builtins for NCI

Megamorphic load builtins are a performance optimization when current
feedback is megamorphic. We can't make this assumption for shared NCI
code though, since feedback in other native contexts may *not* be
megamorphic, and we'd thus miss out on necessary feedback collection.

Bug: v8:8888
Change-Id: I2adc5ef9a6b021b35cf26f975c79228d556bf94f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2546694
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71264}

src/compiler/js-generic-lowering.cc

@@ -234,8 +234,19 @@ void JSGenericLowering::LowerJSStrictEqual(Node* node) {
 }
 
 namespace {
+
+// The megamorphic load builtin can be used as a performance optimization in
+// some cases - unlike the full builtin, the megamorphic builtin does fewer
+// checks and does not collect feedback.
 bool ShouldUseMegamorphicLoadBuiltin(FeedbackSource const& source,
                                      JSHeapBroker* broker) {
+  if (broker->is_native_context_independent()) {
+    // The decision to use the megamorphic load builtin is made based on
+    // current feedback, and is thus context-dependent. It cannot be used when
+    // generating NCI code.
+    return false;
+  }
+
   ProcessedFeedback const& feedback = broker->GetFeedback(source);
 
   if (feedback.kind() == ProcessedFeedback::kElementAccess) {